These tests are important in agriculture since fertilizer application can be fine-tuned if the plants nutrient status is known.
It takes time to dry the samples, send them to the lab, complete the lab-tests, and then return the results to the grower.
For instance, while Minolta uses "SPAD units", the Dualex (produced by METOS® from Pessl Instruments GmbH) uses μg/cm² and ADC uses a Chlorophyll Content Index.
[2] While traditional absorption instruments have been very popular with plant scientists and have proved to work well with broad leaf species, they do have limitations.
Limitations of absorption meters: There are therefore samples which are not suitable for the absorption technique, these include small leaves, most CAM plants, conifer needles, fruit, algae on rocks, bryophytes, lichens and plant structures like stems and petioles.
"[3] The fluorescent ratio chlorophyll content meters use this technique to measure these more difficult samples.
Chlorophyll fluorometers are designed to measure variable fluorescence of photosystem II, or PSII.